An AC LED driving circuit proposed as a scheme for driving LEDs under an AC power condition has advantages, such as a simplified manufacturing process, a low defect rate, and a long lifespan, compared to a Switched Mode Power Supply (SMPS) scheme.
Referring to FIG. 1, FIG. 1 is a diagram showing a conventional, typical AC LED driving circuit. The fundamental principle of such an AC LED driving circuit is to sequentially control current sources.
Further, in order to stably drive a phase-cut dimmer using the AC LED driving circuit of FIG. 1, a current source for causing a hold current required to drive the dimmer to flow must be added to a location between a bridge circuit and a first LED.
FIG. 2 illustrates the conventional AC LED driving circuit. As shown in the drawing, the AC LED driving circuit includes an AC power source 10, a full-half rectifier 20, a plurality of LEDs 31 to 34, a number of current sources 41 to 44 corresponding to the number of LEDs 31 to 34, and an algorithm circuit for controlling the entire circuit, and further includes a dimmer 50 and a bleeding current source 60 for stably operating the dimmer 50.
Further, in the AC LED driving circuit, an I_HOLD current source, that is, the bleeding current source 60, functions to supply a drive current required for the driving of the dimmer to an overall area supplied with power by driving the current required for the driving of the dimmer at the time point when the current sources ILED1˜ILEDn are not driven. Therefore, I_HOLD is set to have a current value required to drive the dimmer, independent of the current sources ILED1˜ILEDn. At the moment at which AC power is increased and then ILED1˜ILEDn are driven, I_HOLD stops its operation. When the AC power is decreased and then the circuit enters an area in which ILED1 is not driven, I_HOLD needs to be driven again.
In other words, the AC LED driving circuit of FIG. 2 needs to perform control such that I_HOLD is turned off at the moment at which ILED1 is driven and I_Hold is driven again at the moment at which ILED1 stops its operation so that I_HOLD and ILED1 are not simultaneously driven. Accordingly, the AC LED driving circuit additionally requires a control circuit for checking the driving time point of ILED1 and turning off I_HOLD. Further, under the condition of ILED1<I_HOLD, input current at the time point at which a first LED group is operated does not reach the current required to drive the dimmer. In this case, a method of supplementing an amount of current necessary for the driving of the dimmer is additionally required.
Referring to FIG. 3A and FIG. 3B, FIG. 3A and FIG. 3B are diagrams showing the current waveforms of the AC LED driving circuit of FIG. 2. As shown in the drawing, FIG. 3A shows that, when ILED1>I_HOLD and both of the two currents are set to current values greater than a minimum current value required to operate the dimmer, I_in represented by the sum of the two currents is also greater than a minimum current value required to drive the dimmer. Thus there is no problem with the driving of the dimmer. However, as shown in FIG. 3B, when the condition of ILED1<I_HOLD is formed while the value of ILED1 becomes less than a minimum current value required to drive the dimmer, I_in is also less than the minimum current value required to drive the dimmer in an area in which the ILED1 is driven. In this case, the dimmer cannot be normally operated.
For reference, in consideration of the recent development level of LEDs, it is expected that LEDs having optical efficiency of 200(lm/W) or more beyond the optical efficiency of LEDs that are currently popularized in markets, that is, 160(lm/W), will be commercialized in several years. This means that LED current is entirely decreased, but a current value required to drive the dimmer is typically maintained at 20 to 40 mA, and thus there is a strong possibility that a situation such as that shown in FIG. 3B will occur.